Humans are continually exposed to a wide variety of chemicals, some of which may act as initiators, promoters, cocarcinogens and/or complete carcinogens. Current evidence suggests that covalent modification of DNA plays a central role in the mechanism of tumor initiation by diverse classes of carcinogenic agents including polycyclic aromatic hydrocarbons (PAH). However, it remains to be determined which DNA-adducts are most important; whether DNA-adducts are removed efficiently or inefficiently thus introducing errors in the DNA; or whether DNA-adducts must persist in the DNA for long periods of time for tumor initiation. The proposed research is designed to investigate the role of PAH DNA-adduct removal and persistence in relation to skin tumor initiation in mice. Mouse skin is a well known target tissue for PAH and is a widely studied model system for skin carcinogenesis. The specific aims of the proposal are to examine, in detail, the time course of binding of benzo[a]pyrene (B[a]P) and 7,12-dimethylbenz[a]anthracene (DMBA) to the epidermal DNA of SENCAR mice over a 21 day period. The rate of formation and disappearance of the individual B[a]P and DMBA DNA-adducts will be monitored over a 21 day time course. We will also determine the role of active DNA adduct removal vs. cell division in the disappearance of B[a]P and DMBA DNA-adducts from epidermal DNA. Furthermore, we will determine the extent of nonsemiconservative or unscheduled DNA synthesis induced by B[a]P and/or DMBA in mouse epidermis. We will also determine the extent of B[a]P (or DMBA) DNA-adduct removal in epidermal cells at differnt stages of differentiation. Finally, in conjunction with the experiments mentioned above, we will determine the effect of inhibiting DNA synthesis at various times during the formation and removal of B[a]P (or DMBA) DNA-adducts on skin tumor initiation. This approach will allow us to test the hypothesis that persistent hydrocarbon DNA-adducts are not necessary for skin tumor initiation in mice.